Our previous studies on the genetic mechanism of metastases have shown that the metastatic phenotype can be conferred upon benign cells through somatic cell hybridization, as well as through DNA transfection. NIH-3T3 cells transfected with either DNA from malignant cells or the isolated Ha-ras oncogene are metastatic in NIH nude mice, but NIH-3T3 cells transfected with normal embryonic DNA or spontaneously transformed in vivo are not metastatci. To further evaluate the role of actobated ras genes in metastases, the DNA levels and expression of the ras specific sequences were measured in both NIH-3T3 transfectants and metastatic NMU-in-duced rat mammary carcinoma containing mutationally activated c-Ha-ras oncogene. Experimental and spontaneous metastases from 3T3/T24 DNA and 3T3/c-Ha-ras transfectants demonstrated a variability in the levels of the Ha-ras- specific DNA sequences which were either lower or higher than the primary tumor. Similarly, a variability was noticed in the levles of Ha-ras specific DNA and RNA sequences in ten lung metastases derived from a primary nitroso-methylurea (NMU)-induced rat mammary carcinoma. The DNA levles ranged from 10% to 300% of that measured in the primary tumor. Three out of 10 metastases expressed very low RNA levels (5-10% of the primary) of the Ha-ras oncogene. Less variability was detected in NMU tumors after long-term passage in vivo, both within each group of primary tumors and the metastases, as well as between the groups of primaries and metastases. Similarly, minor differences were seen in the Ha-ras levels between metastatic and nonmetastatic primary NMU tumors. This work shows that activated ras genes transfected into NIH-3T3 cells can produce metastases in NIH nude mice. Although the ras genes can set in motion the metastatic cascade, their heterogeneous expression in metastases suggests that thy do not play a major role in the growth of metastases.